The Cosmic Radio Background Five Years of an Enigma Jack Singal University of Richmond HEM Workshop...

The Cosmic Radio BackgroundFive Years of an Enigma

Jack SingalUniversity of Richmond

HEM WorkshopKICP, U Chicago

The EM backgrounds

From Hasinger (2000) astroph/0001360

Thermal dust

Last scattering

Starlight

AGN +?

What about over here?

AGN + Star formation (?)

CGB

The Isotropic Radio Sky• Absolutely calibrated single dish all-sky maps at low frequencies (e.g. Haslam et al., 1982, A&A, 47, 1) and ARCADE 2 measurements at higher frequencies (Fixsen et al., 2011, ApJ, 734, 5) agree that there is a significant radio monopole [(~4x) brighter than was once expected]

Haslam 408 MHz all-sky map

• The radio emission has a synchrotron-like spectrum

• It is isotropic on >1 deg scales

The Isotropic Radio Sky• There is a significant synchrotron radio monopole

Haslam 408 MHz all-sky map

• The only dispute is whether it is in the Galactic halo or extragalactic

• You can’t tell the difference just by looking out

The Isotropic Radio SkyHow to distinguish (from maps alone) between Galactic monopole and extragalactic component?

1) Model Galactic radio emission with some function of latitude2) Correlate Galactic radio emission with some other emission of known spatial structure

See Kogut et al. (2011, ApJ, 734, 4)

The Isotropic Radio SkyHow to distinguish between Galactic monopole and extragalactic component?

This modeling places around ¼ of the total isotropic signal into a Galactic halo and the rest into extragalactic

Extragalactic component from this modeling: 67 mk @ 3 GHz and 3.725 K @ 670 MHz

CRB

Ok, assuming extragalactic for a minute…

Our Take• In Singal et al. (2010, MNRAS, 409, 1172) we considered several limitations

1 nG

1 μG

Observed EBL

1) The electrons that produce the radio emission through synchrotron cannot overproduce another background via inverse-Compton – places a lower limit on the magnetic field in the emitting regions


2) If it is a sum of discrete sources, there has to be a lot of them at low fluxes (because radio source counts at resolved fluxes > ~10 μJy only account for ~1/4)

Large radiogalaxies

RQ quasars and ordinary galaxies

Flux limit of surveys

Radio source counts

# of low flux objects needed

Source counts power law slope


3) Ordinary galaxies are numerous enough but if they follow the radio – FIR correlation of the local Universe they would overproduce the infrared background

The Case For Extragalactic• If the CRB is from galaxies we would need the radio – FIR correlation (FRC) to have evolved by a factor of ~5 (intrinsic) between redshift 0 and ~1

Generally consistent conclusions in:

- T. Vernstrom, D. Scott, & J. Wall, 2011, MNRAS, 415, 3641- A. Draper, S. Northcott, & D. Ballantyne, 2011, ApJ, 741:L39

Evidence for the FRC evolving (but not by enough) reported in:

- R. Ivison et al., 2010, MNRAS, 402, 245 "BLAST - The Far-infrared/radio Correlation in Distant Galaxies" - R. Ivison et al., 2010, A&A, 518, L31, "The Radio/Far Infrared Correlation as Probed By Herschel"

The Case Against ExtragalacticThere are many problems with an extragalactic origin

Models for radio source counts that fall too low:

[P(D) analysis to probe radio source counts to very low fluxes]- J. Condon et al., 2012, ApJ, 758, 23

[from infrared counts and applying FRC]- P. Ponente et al., 2011, MNRAS, 419, 691- N. Ysard & G. Lagache, 2012, A&A, 547


- G. Holder (2014, ApJ, 780, 112) "The Unusual Smoothness of the Extragalactic Unresolved Radio Background" Arcminute-scale fluctuations seen at 4.86 and 8.4 GHz are so small that the Radio Background sources must not be clustered like large scale structure

However if the mechanism is z>5, then these constraints are evaded


- J. Cline & A, Vincent, 2013, JCAP, 02, 011C "Cosmological Origin of Anomolous Radio Background" Building on Holder they consider cosmological (high z) injections of photons (can't do) and electrons (needed mag field is too high).

The Case For GalacticR. Subrahmanyan & R. Cowsik, 2013, ApJ, 776, 42 "Is there an unaccounted excess Extragalactic Cosmic Radio Background?" They use a 2-component 5-parameter vs frequency Galactic spatial model for radio emission to get a much higher halo component.

(But see test of this model discussed in A. Kogut’s talk)

The Case For GalacticE. Orlando & A. Strong, 2013, MNRAS, 436, 2127 “Galactic Synchrotron Emission with Cosmic Ray Propagation"

They present new models of Galactic synchrotron obtained by considering CR propagation with GALPROP.

Can reproduce level (?) of radio background at 408 MHz with a 10 kpc cosmic ray halo.

The Case Against Galactic

• Large scale Galactic radio emission is well fit by a csc(b) spatial model, and a correlation of radio with C+ emission at 158 µm agrees

• The same X-ray inverse-Compton argument applies to our Galactic halo, where FR measures show the mag field is ~1 μG (Taylor et al, 2009, ApJ, 702, 1230) and optical/UV flux is higher

• The local Radio-FIR correlation can be applied to the COBE-FIRAS measured level of high latitude infrared Galactic emission and it comes out much lower than the radio background

• Recent paper by Fornengo et al. (2014, JCAP, accepted, arXiv:1402.2218) with detailed Galactic synchrotron modeling

(See talk by A. Kogut for further discussion)

Ok, More Exotic Explanations?If our Galaxy doesn’t work, and other galaxies don’t work…

Annihilating Dark Matter:

N. Fornengo R. Lineros, M. Regis, & M. Taoso, 2011, PRL, 271302 "Possibility of a Dark Matter Interpretation for the Excess in Isotropic Radio Emission Reported by ARCADE" Dark matter in filaments decays into e- for synchrotron but I think the needed magnetic field is too high D. Hooper et al., 2012, PRD, 863, 003H “The Isotropic Radio Background and Annihilating Dark Matter” If annihilating DM is responsible for radio background, it is responsible for part of gamma-ray background too Y. Yang et al, 2013, PRD, 083519, “Contribution of Ultracompact Dark Matter Minihalos to the Isotropic Radio Background” DM in UCMHs can produce CRB but would overproduce X-ray background through inverse-Compton


Or…:

K. Lawson & A. Zhitnitsky, 2012, arXiv: 1210.2400 “Isotropic Radio Background from Quark Nugget Dark Matter” Nuggets of dense matter and antimatter form the Dark Matter. They are heated by annihilations with visible matter and have thermal emission which is essentially flat with frequency. The thermal temperature changes over time and given that and the redshift distribution of the DM density, they calculate a resulting spectrum which can match the CRB. D. Spolyar, P. Bodenheimer, K. Freese, & P. Gondolo, 2009, ApJ, 705, 1031 "Dark Stars: A New Look at the First Stars in the Universe" Dark stars (early stars supported against collapse by annihilating DM) make big black holes which could lead to an explosion of synchrotron in the early Universe


Or Pop III Supernovae

Beirmann et al., 2014, MNRAS, submitted“Cosmic Backgrounds Due to the Formation of the First Generation of Supermassive Black Holes”

Use assumption that radio background is due to synchrotron from Pop III supernovae to recover rate of SMBH formation consistent with SMBH abundance.

Would evade all constraints discussed so far (high redshift, high mag. fields, large # of sources, no radio-FIR because little dust). However disagrees with simulated rates of Pop III supernovae from Wise & Abel (2005, ApJ, 525, 615).

The EM backgroundsAssuming radio monopole is ~1/4 Galactic and the rest CRB

Conclusions• The radio sky has a bright isotropic synchrotron component which is either Galactic or extragalactic

• It must originate from regions of relatively high magnetic field strength (~1 μG)

• If from z<5 it should be from in or near galaxies, and with those galaxies characterized by an evolving radio far-infrared correlation (but this may lead to a background that is clumpier than limits)

• Synchrotron from Pop III supernovae is an intriguing possibility

• If Galactic, it must be either from the halo in regions with a strangely high magnetic field, or more locally with inexplicably low polarization

• The preponderance of the evidence favors the extragalactic scenario (segue to A. Kogut talk)

The Cosmic Radio Background Five Years of an Enigma Jack Singal University of Richmond HEM Workshop...

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